Current controlled motor amplifier system

FIELD OF THE INVENTION

The present invention relates generally to a current controlled motor amplifier system.

BACKGROUND OF THE INVENTION

The conventional approach to transconductance or current controlled motor amplifier design includes current sensing, scaling, error amplification, compensation, and motor drive. One example of such an amplifier is shown in FIG. 1 in which T<b>1</b>, T<b>2</b>, T<b>3</b>, and T<b>4</b> are solid-state switches (usually transistors), M is the electric motor (or one phase of the motor in the case of a multiphase motor), and I is the current in the motor. A reverse biased diode is connected to each switch. The upper and lower terminals of the H bridge are connected to maximum potentials.

The conventional approach is used specifically in areas where significant motor rotational velocities result in high motor back-EMF (electromagnetic force). By sensing the motor current, and developing a current signal proportional to the motor current, the instantaneous motor current can be compared with the commanded motor current. Any difference between the commanded and measured motor current is amplified as an error signal that is the command signal to the motor drive bridge. If the measured current is smaller than the commanded current, then the error signal will command a compensatory increase in motor current. In this way, the output current, and hence an output torque of the motor is proportional to the input command to the amplifier.

The conventional approach is usually well suited to driving motors over a wide range of loads and rotational velocities. A problem with the conventional approach is that it addresses a larger situation than that of the typical force-feedback application, such as wheel or joystick amplifiers in gaming applications, which tend to operate close to stall, or at comparatively low rotational velocities. The breadth of coverage in the conventional approach adds unnecessary complexity to the motor amplifier design for force-feedback applications.

Accordingly, what is needed is an approach to current controlled motor amplifier design that is less complex and more suitable for high fidelity force feedback applications.

SUMMARY OF THE INVENTION

Aspects of a current controlled motor amplifier system are provided. These aspects include a current source motor amplifier comprising current source means on each leg of a top half of the H bridge configuration and switching means on each leg of a bottom half of the H bridge configuration. A motor is coupled to the current source motor amplifier at a center portion of the H bridge configuration. Control circuitry is coupled to the current source motor amplifier for controlling the switching on of the current source motor amplifier for a predetermined time to operate the top half of the H bridge configuration essentially as a linear constant current source and the bottom half of the H bridge configuration in switching mode.

Through the present invention, significant current loop delays associated with conventional approaches are avoided. Further, the present invention provides a less complex and less costly solution that includes overvoltage protection and is more suitable for high fidelity force feedback when changing direction of motor rotation. The present invention also provides for a simple means of motor back EMF compensation. These and other advantages will become readily apparent from the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a diagram of a prior art motor amplifier design.

FIG. 2 illustrates a schematic diagram of a current source motor amplifier in accordance with the present invention.

FIG. 3 illustrates a schematic diagram of an example application of the current source motor amplifier of FIG. 2 in a two axis (X and Y), high current amplifier situation, such as a joystick amplifier in an arcade application.

FIG. 4 illustrates a variant of the current source motor amplifier that includes back EMF compensation and high current motor drive capability.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a current controlled motor amplifier system. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein.

FIG. 2 illustrates a schematic diagram of a current source motor amplifier <b>100</b> in accordance with the present invention. In general, the design is a hybrid between a linear amplifier and a switched-mode amplifier, with the smooth current linearity of a linear amplifier and efficiency that lies between a linear and switched-mode amplifier.

As shown in FIG. 2, the current source motor amplifier <b>100</b> in accordance with the present invention is based on a current sourcing topology. An H bridge includes current sources <b>102</b><i>a </i>and <b>102</b><i>b </i>on each leg of the top of the bridge and switching mechanisms <b>104</b>, e.g., N-channel FETs (field effect transistor), on each leg of the bottom of the bridge with the motor <b>106</b> lying in the horizontal portion of the H bridge. One current source is used for each of the two force directions of the motor <b>106</b>, e.g., current source <b>102</b><i>a </i>for a forward direction, and current source <b>102</b><i>b </i>for a reverse direction. Appropriate biasing via base resistance Rb and transistors <b>110</b>, operates with transistors <b>108</b><i>a </i>and <b>108</b><i>b</i>, e.g., PNP bipolar junction transistors (BJTs), to provide current sources <b>102</b><i>a </i>and <b>102</b><i>b</i>. A pair of the forward biased diodes <b>112</b> prevents the base voltage of the current sources from falling below the supply voltage minus two voltage drops, as is well appreciated by those skilled in the art.

Thus, for a nominal 12 V (volt) supply, the base voltage of transistor <b>102</b> does not fall below 12 V−2*0.7 V=10.6 V. This controls the emitter voltage to one diode drop above the base voltage, or 10.6 V+0.7 V=11.3 V. The voltage drop across the current controlling resistor, Rs, from the supply to the emitter is therefore controlled to 0.7 V nominally. This fixed voltage drop allows a fixed current to pass through the current controlling resistor. The base resistance Rb=(Vs−2*0.7−Vsat)/(Imax/β)/Ksf; where Vs is the maximum supply voltage, Vsat is the NPN collector to emitter saturation voltage of transistor <b>110</b>, Imax is the peak PNP emitter current, B is the minimum PNP device gain, and Ksf is the design safety factor. For a typical application, Vs=12 V, Vsat=0.4 V, Imax=750 mA (milliAmps), β=50, and Ksf=2. Thus, Rb=approximately 340 ohms, or 330 ohms as a nominal value. The source resistance, Rs,=0.7 V/[(1−1/β)Imax] or approximately 0.952 ohms, nominally 1 ohm.

In operation, a current source <b>102</b> is turned on by pulling the base of the appropriate transistor low. By way of example, control circuitry <b>114</b> may include a controller capable of sending direction signals to a multiplexer (MUX) for the selection of which current source is turned on, and, further, the control of which of the switching transistors is allowed to be modulated (switched on) by a PWM (pulse width modulate) force command. Preferably, only one current source <b>102</b> and switching mechanism <b>104</b> in the bridge are active at a time, where the one current source <b>102</b> and switching mechanism <b>104</b> that are active lie diagonally from one another in the bridge. In operation, the current is sourced from the transistor of the selected current source <b>102</b>, flows through the motor header to the motor winding and returns to ground through the selected switching mechanism <b>104</b> on the bottom of the H bridge. Since the gate of the transistor of the switching mechanism <b>104</b> is modulated, it conducts for only the portion of the PWM period in which the signal is high. Since a return path to the power supply is only available for the positive portion on the PWM cycle, the current source provides current only for that portion of the PWM cycle, thus controlling the average current through the motor and thereby the torque generated by the motor, as is well appreciated by those skilled in the art.

FIG. 3 illustrates a schematic diagram of an example application of the current source motor amplifier of FIG. 2 in a two axis (X and Y), high current amplifier situation, such as a joystick amplifier in an arcade application. The diagram of FIG. 3 illustrates an example embodiment of the control circuitry that includes a MUX <b>120</b><i>a </i>and <b>120</b><i>b </i>on each of the X and Y axes. The MUX <b>120</b><i>a </i>or <b>120</b><i>b </i>is required in normal operation and operates by channeling the PWM signal from a controller <b>122</b> to either the right or left switching mechanism <b>104</b> based on the direction signal for the axis from the controller. The opposite switching mechanism is grounded appropriately based on the sense of the direction signal. Likewise the MUX <b>120</b><i>a </i>or <b>120</b><i>b </i>turns on one of the current source transistors on the top of the bridge while the enable signal from the controller is low.

While operating successfully as a current source amplifier, a further aspect is the ability to drive the motor with either a clockwise or counter-clockwise torque depending upon whether the motor is rotating clockwise or counter-clockwise, i.e., four quadrant operation. The amplifier therefore makes a smooth transition between forward driving the motor and dynamically braking the motor. This occurs when the current source amplifier senses motor back-EMF, such as when the actuator is driven by the user and the motor is rotating in a direction that is opposed to the commanded torque direction, which in turn causes the motor to act as a generator that produces an output voltage proportional to the rotational velocity of the motor.

FIG. 4 illustrates a variant of the current source amplifier that is intended for applications, such as steering wheel force-feedback devices, that use high mechanical gearing between the motor and steering wheel, where the effects of the motor velocity require compensatory circuitry. The current source amplifier is designed to protect itself against back EMF generated by the user back-driving the motor through a reduction in the commanded motor current when the onset of the body diode conduction in the switching transistor <b>130</b> is detected by transistor <b>132</b>. Transistor <b>132</b> begins to conduct when its emitter voltage drops below the biased voltage of the base, which is close to ground. Since the anode of the body diode of transistor <b>130</b> is at ground potential, when it is induced to conduct by the motor, its cathode and hence the emitter of transistor <b>132</b> is brought below ground, causing the collector of transistor <b>132</b> to conduct.

Note that the drive signal for the switching FETs <b>130</b> and <b>134</b> is capacitively coupled by capacitors <b>136</b> and <b>138</b> and weakly driven in parallel by resistors <b>140</b> and <b>142</b>. This ensures that the FETs <b>130</b> and <b>134</b> are able to conduct for a minimum duration at the leading edge of the PWM command signal from MUX <b>144</b>. This further ensures that no current pulses are dropped, so that the amplifier always operates at a frequency that is above the audible range, and thus, prevents audible noise from being generated in the motor. The weak resistive coupling of the drive signal also ensures that after the initial turn-on pulse, the back EMF compensation circuitry can effectively regulate the duration of the commanded pulse and hence the induced damping generated by the motor back EMF.

When transistor <b>132</b>, for example, detects body diode conduction in switching transistor <b>130</b>, the collector of <b>132</b> then discharges capacitor <b>138</b> and prematurely terminates the command signal that drives the gate of switching transistor <b>134</b>. The net result is that body diode conduction in one FET on the bottom of the bridge causes a compensatory reduction in the complementary FET on the opposite side of the bridge that virtually eliminates the electrical damping induced by the motor back EMF when the motor is back-driven. This innovation eliminates the sensation of resistance that is felt by the user and detracts from the haptic experience. The same functionality could likewise be accomplished with a bipolar transistor and a diode placed from emitter to collector similar to the body diode in the FET from source to drain, as is well appreciated by those skilled in the art.

FIG. 4 illustrates an additional aspect of the design that is different from the embodiment in FIG. <b>3</b>. Current sensing transistor <b>156</b> provides the current source regulation that is independent of the base to emitter voltage of the power transistors <b>158</b> and <b>160</b>. This is important in higher current applications as the base to emitter voltage of the power transistors varies significantly as a function of collector current and much more than the emitter to base voltage drop of the small signal transistor that is used for current sensing. By biasing the base of transistor <b>156</b> with diodes <b>162</b> and <b>164</b>, the collector current of transistor <b>156</b> depends almost exclusively on the voltage drop across resistor <b>166</b>, and hence the collector current of the enabled power transistor <b>158</b> or <b>160</b>.

In still a further aspect, overvoltage protection is readily achieved in the current source amplifier. As shown in the diagram of FIG. 3, in an exemplary embodiment, a zener diode <b>124</b> is used to detect an over-voltage condition and trigger a series of events that protect the bridge. Once detected, a high logic level voltage places the outputs of the MUX <b>120</b><i>a </i>or <b>120</b><i>b </i>into a tri-state condition. Secondly, both of the switching transistors on the bottom of the bridge are turned on and the current sources on the top of the bridge are disabled. Accordingly, a low impedance current path is placed across the motor through the ground plane and shorts out the motor, which is the source of voltage. When the supply voltage drops, the overvoltage protection circuit releases the MUX <b>120</b> from its tri-state condition, and normal operation continues. In practice, when the user is back-driving the motor with sufficient velocity to generate an overvoltage condition, the amplifier will rapidly (on the order of a few hundred microseconds) toggle between normal mode and overvoltage protection mode such that the motor voltage is clamped.

In the exemplary circuit in FIG. 4, a signal conditioning circuit, including diode <b>146</b>, transistor <b>148</b>, transistor <b>150</b>, and associated passive components, generates a 5 volt signal at the collector of transistor <b>148</b> that both causes the output of MUX <b>144</b> to drive its outputs low, disables the top of the bridge through diodes <b>152</b> and <b>154</b> and turns on both FETs <b>130</b> and <b>134</b> on the bottom of the bridge. Thus, rather than using a separate crowbar overvoltage protection circuit, as is used in prior art, the bridge power components can provide the overvoltage protection function. An alternative embodiment uses the transistors on the top of the bridge to likewise circulate induced motor current through the positive supply rail.

As demonstrated by these aspects, with the current source amplifier of the present invention, the top half of the bridge is essentially a linear constant current source and the bottom half of the bridge operates in switching mode. This gives the current source amplifier the clean linearity of a linear amplifier but with a higher efficiency that is more characteristic of a pure switch-mode amplifier. Amplifier linearity is also maintained when used with high power PNP transistors. Further, the current source amplifier operates in current mode and is a transconductance amplifier, but does not require classic closed loop current control, saving considerable complexity and cost while avoiding current loop delays. Additional reduction in complexity and cost results from achieving overvoltage protection via the switching transistors on the bottom of the bridge, rather than requiring a separate crowbar circuit, as is needed in standard transconductance amplifiers. Thus, the design is scalable, simple, and low cost.

This simplified design also improves the dynamic range of the motor by decreasing the minimum command at which proportional output current is provided. In a 750 mA application, the design provides for linear output current for current commands of as little as 5 mA and non-zero output current for current commands of as little as one part in 1024 of PWM command. This provides a clean transition through a direction reversal without the notch sensation of dropout through the zero crossing that is associated with other low cost amplifier designs.

From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the novel concept of the invention. It is to be understood that no limitation with respect to the specific methods and apparatus illustrated herein is intended or should be inferred. For example, although the present invention is described in terms of current sources on the top of the bridge, an alternate embodiment could use current sinks on the bottom of the bridge to achieve the aspects of the controlled motor amplifier as described herein. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.